Add the ability to right-click a node and set its colour

[desplesda]

Release v2.2.77 added the ability to set a colour label on nodes through the use of a color header, but that feature requires you to manually type in the header.

We should expose the ability to right-click on a node in the graph view and choose a colour from a context menu that appears. Once a colour has been selected, the extension can add or modify the header based on the selection.

For bonus points, if possible, it'd be nice to make the right-click menu show a tickbox next to the currently selected colour.